DESCRIPTION (Verbatim from the Applicant's Abstract): Dendritic spines are tiny protrusions that stud the surface of principal neurons throughout the brain. They are the primary sites of excitatory synapses, the sites of communication between neurons. The goal of the proposed work is to ascertain the role of synaptic activity in the formation and structure of dendritic spines and their synapses. It had been thought that spines are generated when synaptic activity is enhanced. Our findings show however that spines and synapses are stable during long term enhancement of synaptic activity. Instead we have discovered that dendrites of mature neurons become more spiny within hours after synapses are inactivated. The specific aims of this project are: 1) Establish how soon after inactivation dendrites of mature hippocampal neurons become more spiny. 2) Assess whether dendrites become less spiny after reinstatement of activity. 3) Determine how inactivation and reactivation affect the structure of spines, synapses, and their associated astroglial processes. 4) Determine whether the additional spine synapses are functional. In Aims 1, 2, and 4 quantitative confocal microscopy will be used with field and whole-cell recordings in hippocampal slices from adult rats. In Aim 3 quantitative serial electron microscopy will be used to answer a series of related questions including: i) Do all of the spiny protusions that are detected with confocal microscopy have synapses? ii) Do the additional synapses occur on pre-existing axonal boutons? iii) Is synaptic structure modified during inactivation or reactivation? iv) Are the calcium-regulating organelles in dendritic spines modified by activity? v) Does activity modify the degree of astroglial ensheathment of spine synapses? The reported progress and proposed research present an important new way of thinking about spines and synapses in the mature brain. They suggest that mature neurons maintain an optimal level of activation by regulating spine and synapse number. They could explain why spines are lost after excessive activity, for example, during epileptic seizures. They are also relevant to understanding how spines and synapses might be generated in the mature brain at times when overall brain activity is low. This outstanding renewal application formulates a new and novel view of dendritic spine plasticity that runs counter to established dogma but is consistent with recent findings of other innovative workers in the field, and extends work carried out during the prior application period into significant new areas. Light microscopic and ultrastructural findings as well as neurophysiological data strongly suggest that spine density exhibits multiple states, with certain levels of neuronal activity stabilizing both spine and synaptic number, while increased activity down-regulates both and decreased activity actually up-regulates both, as if neurons strive to maintain stable "activation," irregardless of variations in input activity.